Systems and methods for increasing cable modem system bandwidth efficiency

ABSTRACT

A cable modem termination system measures signal qualities of upstream transmissions associated with one or more cable modems. The system monitors the measured upstream signal qualities, and selectively commands at least one of the one or more cable modems to switch between upstream channels based on the signal quality monitoring.

CROSS REFERENCE TO RELATED APPLICATION

The instant application claims priority from provisional application No.60/409,982, filed Sep. 12, 2002, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to cable modem systems and, moreparticularly, to systems and methods for improving bandwidth efficiencyin cable modem systems.

2. Description of Related Art

In conventional cable modem systems, a cable modem termination system(CMTS) at the headend typically services multiple cable modems (CMs).The CMTS transmits data and messages to the CMs on a downstreamfrequency and receives data bursts from the CMs on different upstreamfrequencies. Conventionally, CMTSs' upstream receivers are set toreceive upstream signals from the CMs based on the capabilities of thelowest performing CM. Thus, even though some CMs may have higherperformance capabilities, all CMs will be set to transmit at thesettings of the least capable CM. With all CMs set to transmit at thesettings of the least capable CM, the available upstream bandwidth isused inefficiently. The CMs with higher performance capabilities willuse more bandwidth than if they transmitted using their higherperformance capabilities. Conventional cable modem systems, thus,inefficiently use available upstream bandwidth.

Therefore, there is a need in the art to more efficiently use upstreambandwidth in cable modem systems.

SUMMARY OF THE INVENTION

Systems and methods consistent with the principles of the inventionaddress this and other needs by altering transmission characteristics ofmodems of a cable modem system to improve bandwidth utilization. Systemsand methods consistent with the principles of the invention may monitorupstream transmission quality and command cable modems to alter theirtransmission characteristics to improve transmission rate and, thus,increase bandwidth utilization. For example, altering the modulationscheme a modem uses (e.g., QPSK to 16QAM) may substantially improve themodem's transmission rate. By “moving” one or more modems from underperforming transmission settings to better performing transmissionsettings, available bandwidth of the cable modem system may be used moreefficiently.

In accordance with one aspect of the invention as embodied and broadlydescribed herein, a method of altering modem transmissioncharacteristics includes setting a modem to transmit on a first upstreamchannel using first transmission characteristics. The method furtherincludes monitoring a quality of the modem upstream transmissions on thefirst upstream channel. The method also includes setting the modem totransmit on a second upstream virtual channel using second transmissioncharacteristics based on the monitored quality.

In another implementation consistent with principles of the invention, amethod of controlling transmission characteristics of cable modemsincludes monitoring upstream transmission quality of one or more cablemodems. The method further includes commanding at least one of the oneor more cable modems to change its transmission characteristics based onthe monitored quality.

In still another implementation consistent with principles of theinvention, a method of changing virtual upstream channels in a cablemodem system includes monitoring upstream signal qualities associatedwith one or more cable modems. The method further includes selectivelyswitching at least one of the one or more cable modems between virtualupstream channels based on the signal quality monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, explain the invention. In the drawings,

FIG. 1 is a diagram of an exemplary network in which systems and methodsconsistent with the principles of invention may be implemented;

FIG. 2 is a diagram of an exemplary cable modem termination system(CMTS) according to an implementation consistent with the principles ofinvention;

FIG. 3 is a diagram of an exemplary cable modem (CM) according to animplementation consistent with the principles of invention;

FIG. 4 is a diagram of exemplary upstream/downstream communicationsbetween a CMTS and multiple cable modems according to an implementationconsistent with the principles of invention;

FIG. 5 is a diagram of an exemplary upstream channel descriptoraccording to an implementation consistent with the principles of theinvention;

FIG. 6 is a flow chart illustrating an exemplary CMTS upstreamtransmission quality monitoring process according to an implementationconsistent with the principles of the invention; and

FIG. 7 is a flowchart illustrating an exemplary CM transmissioncharacteristic parameter alteration process according to animplementation consistent with the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers may be used indifferent drawings to identify the same or similar elements. Also, thefollowing detailed description does not limit the invention. Instead,the scope of the invention is defined by the appended claims andequivalents.

Systems and methods consistent with the principles of the inventionimplement mechanisms for moving cable modems in a cable modem systemfrom under performing transmission settings to better performingtransmission settings to improve upstream bandwidth utilization.Different cable modems of the system may, thus, transmit using differenttransmission characteristics, such as, for example, a differentmodulation or symbol rate. A CMTS, consistent with the principles of theinvention, may monitor transmission quality from each of the modems ofthe cable modem system to ensure that the transmissions from each cablemodem maintain a sufficient level of quality. If transmissions from anyone cable modem does not meet an expected level of quality, orsufficiently exceeds the expected level of quality, the CMTS may commandthe cable modem to change its transmission characteristics to eitherimprove the transmission quality or improve the cable modem'sperformance (e.g., transmission rate), respectively.

Exemplary Network

FIG. 1 is a diagram of an exemplary network 100 in which systems andmethods consistent with the principles of the invention may beimplemented. Network 100 may include sub-network 105 and cablesub-network 110 interconnected via a CMTS 115. Host(s) 120 and server(s)125 may connect with sub-network 105 via any type of link, such as, forexample, wired, wireless or optical connection links. Sub-network 105can include one or more networks of any type, including a Public LandMobile Network (PLMN), Public Switched Telephone Network (PSTN), localarea network (LAN), metropolitan area network (MAN), wide area network(WAN), Internet, or Intranet. The one or more networks may alternativelyinclude packet-switched sub-networks, such as, for example, GeneralPacket Radio Service (GPRS), Cellular Digital Packet Data (CDPD), andMobile IP sub-networks.

Cable sub-network 110 may include a coaxial or hybrid opticalfiber/coaxial (HFC) cable network. Cable modems 130-1 through 130-N mayinterconnect with cable sub-network 110 via coaxial cable/optical fiber.Each cable modem 130 couples with a respective Customer PremisesEquipment (CPE) 135. Each CPE 135 may include a television, a computer,a telephone, or any other type of equipment that can receive and/or senddata via cable network 110.

CMTS 115 may transmit data received from host(s) 120 or server(s) 125 onone or more downstream channels via cable network 110 to cable modems130. Cable modems 130 may receive the downstream transmissions and passthe demodulated transmissions on to respective CPEs 135. Cable modems130 may further receive data from respective CPEs 135, modulate thedata, and transmit the data on one or more upstream channels to CMTS 115via cable network 110. CMTS 115 may forward the data, via network 105,to host(s) 120 or server(s) 125.

It will be appreciated that the number of components illustrated in FIG.1 is provided for explanatory purposes only. A typical network mayinclude more or fewer components than are illustrated in FIG. 1.

Exemplary Cable Modem Termination System

FIG. 2 illustrates a diagram of an exemplary CMTS 115 according to animplementation consistent with the principles of the invention. CMTS 115may include one or more processing units 205, a memory 210, acommunication interface 215, an upstream/downstream communicationinterface 220, and a bus 225.

Processing unit 205 may perform data processing functions for datatransmitted/received via communication interface 215 to/from sub-network105, and data transmitted/received via upstream/downstream communicationinterface 220 to/from cable network 110. Memory 210 may include RandomAccess Memory (RAM) that provides temporary working storage of data andinstructions for use by processing unit 205 in performing control andprocessing functions. Memory 210 may additionally include Read OnlyMemory (ROM) that provides permanent or semi-permanent storage of dataand instructions for use by processing unit 205. Memory 210 can alsoinclude large-capacity storage devices, such as a magnetic and/oroptical recording medium and its corresponding drive.

Communication interface 215 may include conventional circuitry wellknown to one skilled in the art for transmitting data to, or receivingdata from, sub-network 105. Upstream/downstream communication interface220 may include transceiver circuitry well known to one skilled in theart for transmitting data bursts on downstream channels, and receivingdata bursts on upstream channels, via cable sub-network 110. Suchtransceiver circuitry may include amplifiers, filters,modulators/demodulators, interleavers, error correction circuitry, andother conventional circuitry used to convert data into radio frequency(RF) signals for transmission via cable network 110, or to interpretdata bursts received from cable modems 130 via cable network 110 as datasymbols.

Bus 225 interconnects the various components of CMTS 115 to permit thecomponents to communicate with one another.

Exemplary Cable Modem

FIG. 3 illustrates a diagram of an exemplary CM 130 according to animplementation consistent with the principles of the invention. CM 130may include a processing unit 305, a memory 310, a CPE interface 315, anupstream transmitter 320, a downstream receiver 325, and a bus 330.

Processing unit 305 may perform data processing functions for datareceived via downstream receiver 325 and data transmitted via upstreamtransmitter 320. Memory 310 may include RAM that provides temporaryworking storage of data and instructions for use by processing unit 305in performing control and processing functions. Memory 310 mayadditionally include ROM that provides permanent or semi-permanentstorage of data and instructions for use by processing unit 305. Memory310 can also include large-capacity storage devices, such as a magneticand/or optical recording medium and its corresponding drive.

CPE interface 315 may include circuitry well known to one skilled in theart for interfacing with a CPE 135. Upstream transmitter 320 may includecircuitry well known in the art for transmitting on an upstream channel.For example, upstream transmitter 320 may include amplifiers, filters,modulators, interleavers, error correction circuitry, and otherconventional circuitry used to convert data into RF signals fortransmission via cable sub-network 110. Downstream receiver 325 mayinclude circuitry well known to one skilled in the art for receivingdata bursts on a downstream channel. For example, downstream receiver325 may include amplifiers, filters, demodulators and other conventionalcircuitry used to interpret data bursts received from CMTS 115 as datasymbols.

Bus 330 interconnects the various components of CM 130 to permit thecomponents to communicate with one another.

Exemplary Downstream/Upstream Communication

FIG. 4 illustrates exemplary upstream and downstream communicationbetween a CMTS 115 and multiple CMs 130 according to an implementationconsistent with the principles of the invention. As illustrated in FIG.4, CMTS 115 and CMs 130-1 through 130-N interconnect via downstream RFchannels 405 and upstream RF channels 410 of cable network 110. Eachdownstream channel 405 and upstream channel 410 may include a differentfrequency. CMTS 115 may transmit messages and data to each CM 130 on adownstream channel 405 and may receive transmission from each CM 130 viaan upstream channel 410. Each upstream channel 410 may include multiple“virtual” channels. Each virtual upstream channel may include a timedivision multiplexed (TDM) timeslot of the upstream channel frequency.Each virtual upstream channel may further be associated with differenttransmission characteristics of cable modems 130. Such differenttransmission characteristics may include a different channel profile,such as different TDM timeslot size, symbol rate, frequency, pre-amblepattern, and/or burst profile. The different burst profile may include adifferent modulation, pre-amble length, data block size (e.g.,Reed-Solomon block size), error correction (e.g., Reed-Solomon errorcorrection), scrambling or encryption, encoding (e.g., differentialencoding), maximum burst size, and/or guard time size.

Upstream channels 410 from cable modems 130-1 through 130-N may, thus,include frequency bandwidth divided into multiple channels, with eachchannel possibly further time division multiplexed into multiple virtualupstream channels. CMTS 115 may monitor a quality of the transmissionsfrom each CM 130 on upstream channels 410. Based on this monitoring,CMTS 115 may command one or more CMs 130 to change their transmissioncharacteristics, such as changing their channel and/or burst profile.For example, CMTS 115 may command CM 130-1 to increase its performance(i.e., bit rate) by changing from quadrature phase shift keying (QPSK)modulation to 16 quadrature amplitude modulation (16QAM) (or to 8QAM,32QAM or 64QAM). Such a change may increase CM 130-1's transmission ratefrom, for example, approximately 2 Mbps to approximately 10 Mbps. Inother implementations, for example, CMTS 115 may command CM 130-1 tochange from 8QAM to 32 QAM, from 16QAM to 64QAM, etc. to increase CM130-1's performance. As another example, CMTS 115 may command CM 130-Nto increase its symbol rate. Thus, if CM 130-N is using QPSK andtransmitting at approximately 2 Mbps, increasing the symbol rate mayincrease the bit rate.

CMTS 115 may issue commands to CMs 130 to instruct them to select one ofmultiple upstream channel descriptors (UCDs). The multiple UCDs each maydescribe different upstream transmission characteristics that are to beused by CMs 130 for transmitting on an upstream channel 410.

Exemplary Upstream Channel Descriptor

FIG. 5 illustrates an exemplary upstream channel descriptor (UCD) 500,one or more of which may be periodically transmitted from CMTS 115 toCMs 130, according to an implementation consistent with the principlesof the invention. UCD 500 may include a header 505 and a message payload510. Header 505 may include conventional overhead data for the use ofany type of medium access control protocol.

Message payload 510 may include an upstream channel identifier 515, aconfiguration change count 520, a time-slot size 525, a downstreamchannel identifier 520 and channel/burst descriptors 535. Upstreamchannel identifier 515 may identify the upstream channel that isassociated with this UCD 500. Configuration change count 520 mayindicate when any values of this UCD 500 change. If the value of count520 remains the same, a receiving CM 130 can conclude that the fields ofUCD 500 have not changed, and may disregard the remainder of themessage. Time-slot size 525 may indicate the size T of the time-slot forthe upstream channel identified by upstream channel identifier 515. Tmay include integer multiples of 2 (e.g., T=2M).

Downstream channel identifier 530 may indicate the downstream channel onwhich UCD 500 has been transmitted. Burst/channel descriptors 535 mayindicate channel and burst profiles for CM transmission on the channelidentified by upstream channel identifier 515. The channel profile mayinclude symbol rate, frequency and pre-amble pattern. The burst profilemay include modulation (e.g., QPSK, 16AM, 8QAM, 32QAM, 64QAM), pre-amblelength, data block size, error correction, scrambling or encryption,encoding, maximum burst size, and guard time size.

Exemplary CM Transmission Quality Monitoring Process

FIG. 6 illustrates an exemplary process for monitoring CM transmissionquality in a manner consistent with the principles of the invention. Asone skilled in the art will appreciate, the method exemplified by FIG. 6can be implemented as a sequence of instructions and stored in memory210 of CMTS 115 for execution by processing unit(s) 205.

The exemplary process may begin with CMTS 115 periodically broadcastingmultiple UCDs on one or more downstream channels 405, with each of themultiple UCDs describing different upstream transmission characteristics[act 605]. For example, each UCD 500 may include a different upstreamchannel identifier 515. Each UCD may further include differentchannel/burst descriptors 535. CMTS 115 may also transmit a bandwidthallocation message for each upstream channel 410 [act 610]. Eachbandwidth allocation message may define transmission opportunities on anassociated upstream channel 410, such as, for example, available timeslots over which a CM 130 may transmit.

CMTS 115 may then monitor the transmission quality of each upstreamtransmission from a corresponding CM 130 [act 615]. CMTS 115 may monitorquality parameters, such as, for example, bit-error-rate orsignal-to-noise ratio, using well-known circuitry withinupstream/downstream communication interface 220.

CMTS 115 may determine whether the upstream transmission quality of anyCM 130 is inadequate [act 620]. Transmission quality measurements foreach upstream channel may be compared with a single upstream qualityparameter to determine whether the measured transmission quality doesnot meet the quality requirements. Alternatively, transmission qualitymeasurements for each upstream channel may be compared with differentupstream quality parameters associated with each channel. If theupstream quality of any CM(s) 130 is inadequate, CM(s) 130 with theinadequate upstream transmission quality may be commanded to use aselected UCD that include more robust transmission characteristics [act625]. For example, one UCD may include a burst profile with modulationset to QPSK, whereas another UCD may include a channel profile withmodulation set to 16QAM, 8QAM, 32QAM or 64QAM. CMTS 115 may command CM130 to select a specific UCD that includes a description of more robustQPSK modulation.

If the upstream transmission quality of any CM 130 is sufficientlyadequate, CMTS 115 may further determine whether the upstreamtransmission quality of a CM 130 is greater than a pre-selected qualitythreshold [act 630]. If not, the exemplary process may return to act 605above. If the upstream transmission quality of any CM 130 is greaterthan the quality threshold, then the appropriate CM(s) 130 may becommanded to use a selected UCD with higher performance transmissioncharacteristics [act 635]. For example, if a monitored CM 130 has a biterror rate less than a pre-selected maximum bit error rate, than CMTS115 may command that CM 130 to use a UCD that includes channel/burstdescriptors 535 that specify, for example, 16QAM modulation instead ofthe CM 130′s current QPSK modulation. The exemplary process of acts605-635 may be selectively repeated to, for example, maximize theperformance of any CM 130 (e.g., maximize bit rate) while maintainingadequate upstream signal quality.

Exemplary CM Transmission Characteristic Alteration Process

FIG. 7 illustrates an exemplary cable modem transmission characteristicalteration process according to an implementation consistent with theprinciples of the invention. As one skilled in the art will appreciate,the process exemplified by FIG. 7 can be implemented as a sequence ofinstructions and stored in memory 310 of CM 130 for execution byprocessing unit 305.

The exemplary process may begin with a CM 130 either being newlyconnected to cable sub-network 110, or being re-booted. CM 130 mayperiodically receive multiple UCDs 500 on a downstream channel 405 [act705]. Each UCD 500 may specify different transmission characteristicsfor CM 130, such as, for example, different channel/burst descriptors535. Each UCD 500 may further specify a different upstream virtualchannel via upstream channel identifier 515. CM 130 may also receivebandwidth allocation messages corresponding to each of the received UCDs[act 710]. One of the UCDs may be selected and CM 130 may then transmiton an upstream channel 410 using transmission characteristics identifiedby the selected UCD [act 715]. CM 130 may select one of the multipleUCDs randomly or according to any other criteria.

A determination may be made whether a “change transmissioncharacteristics” command has been received from CMTS 115 on a downstreamchannel 405 [act 720]. If not, the exemplary process may return to act705 above. If a change transmission characteristics command has beenreceived, then CM 130 may select a UCD 500, as commanded by CMTS 115,from the multiple received UCDs [act 725]. The multiple UCDs 500 may bereceived periodically from CMTS 115 and, thus, CM 130 may select a UCDcommanded by CMTS 115 from the most recently received UCDs 500.

CM 130 may transmit on an upstream channel 410 using transmissioncharacteristics identified by the selected UCD [act 730]. The selectedUCD may include a different channel and/or burst profile inchannel/burst descriptors 535. The different channel and/or burstprofile may specify, for example, a different modulation scheme or adifferent symbol rate that increases the transmission rate of CM 130.For example, the selected UCD may change CM 130 from QPSK to 16QAMmodulation, thus, increasing the transmission rate from, for example,approximately 2 Mbps to approximately 10 Mbps. The exemplary process ofFIG. 7 may be selectively repeated by CMs 130-1 through 130-N so that asmany CMs 130 as possible can move to better performing (i.e., highertransmission rate) transmission characteristics. Bandwidth utilization,thus, may be improved using the exemplary processes of FIGS. 6-7consistent with the principles of the invention.

CONCLUSION

Consistent with the principles of the present invention, processes maybe implemented that selectively alter the transmission characteristicsof one or more modems of a cable modem system to improve bandwidthutilization. Systems and methods consistent with the principles of theinvention may monitor upstream transmission quality and command cablemodems to alter their transmission characteristics to improvetransmission rate and, thus, increase bandwidth utilization. Forexample, altering the modulation scheme a modem uses (e.g., QPSK to16QAM, QPSK to 32QAM, etc.) may substantially improve the modem's bitrate. By “moving” one or more modems from under performing transmissionsettings to better performing transmission settings, available bandwidthof the cable modem system may be utilized more efficiently.

The foregoing description of embodiments of the present inventionprovides illustration and description, but is not intended to beexhaustive or to limit the invention to the precise form disclosed.Modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Forexample, certain portions of the invention have been described asexecuted as instructions by one or more processing units. However,implementations, other then software implementations, may be used withthe present invention, including, for example, hardware implementationssuch as application specific integrated circuits, field programmablegate arrays, or combinations of hardware and software. Furthermore,instead of, or in addition to, channels being time division multiplexedinto multiple virtual upstream channels, channels that include portionsof a frequency bandwidth may be code division multiplexed (e.g., CDMA)into multiple virtual upstream channels. While series of acts has beendescribed in FIGS. 6 and 7, the order of the acts may vary in otherimplementations consistent with the present invention. Also,non-dependent acts may be performed in parallel.

No element, act, or instruction used in the description of the presentapplication should be construed as critical or essential to theinvention unless explicitly described as such. Also, as used herein, thearticle “a” is intended to include one or more items. Where only oneitem is intended, the term “one” or similar language is used. The scopeof the invention is defined by the claims and their equivalents.

1-41. (canceled)
 42. A method comprising: setting a modem to transmit ona first upstream channel on a first frequency using first transmissioncharacteristics; monitoring a quality of upstream transmissions from themodem on the first upstream channel; and setting the modem to transmiton a second different upstream channel on a second different frequencyusing second transmission characteristics based on the monitoredquality.
 43. The method of claim 42, where setting the modem to transmiton a second different upstream channel further comprises: determiningwhether the quality of the modem upstream transmissions is inadequate;and setting the second transmission characteristics to more robusttransmission characteristics based on the determination.
 44. The methodof claim 42, where setting the modem to transmit on a second differentupstream channel further comprises: determining whether the quality ofthe modem upstream transmissions is greater than a threshold; andsetting the second transmission characteristics to better performingtransmission characteristics based on the determination.
 45. The methodof claim 43, where the first transmission characteristics comprise atleast one of 16 quadrature amplitude modulation (16QAM), 8QAM, 32QAM, or64QAM, and the second transmission characteristics comprise quadraturephase shift keying (QPSK) modulation.
 46. The method of claim 44, wherethe first transmission characteristics comprise quadrature phase shiftkeying (QPSK) modulation and the second transmission characteristicscomprise at least one of 16 quadrature amplitude modulation (16QAM),8QAM, 32QAM, or 64QAM.
 47. The method of claim 42, where the firstupstream channel comprises a first time division of the first frequency.48. The method of claim 47, where the second different upstream channelcomprises a second time division of the second different frequency. 49.The method of claim 42, where the quality comprises at least one ofbit-error-rate or signal-to-noise ratio.
 50. A cable modem terminationsystem, comprising: a memory to store instructions; a communicationinterface to: receive transmissions comprising first transmissioncharacteristics from a modem on a first upstream channel on a firstfrequency, and measure a quality of the received upstream transmissionsfrom the modem; and a processor to execute the instructions in thememory to: monitor the measured quality of the received transmissions,and send a message, via the communication interface, instructing themodem to transmit on a second different upstream channel on a seconddifferent frequency using second transmission characteristics based onthe monitored quality.
 51. The system of claim 50, the processor furtherto: determine whether the quality of the modem upstream transmissions isinadequate; and set the second transmission characteristics to morerobust transmission characteristics based on the determination.
 52. Thesystem of claim 50, the processor further to: determine whether thequality of the modem upstream transmissions is greater than a threshold;and set the second transmission characteristics to better performingtransmission characteristics based on the determination.
 53. The systemof claim 51, where the first transmission characteristics comprise atleast one of 16 quadrature amplitude modulation (16QAM), 8QAM, 32QAM, or64QAM, and the second transmission characteristics comprise quadraturephase shift keying (QPSK) modulation.
 54. The system of claim 52, wherethe first transmission characteristics comprise quadrature phase shiftkeying (QPSK) modulation and the second transmission characteristicscomprise at least one of 16 quadrature amplitude modulation (16QAM),8QAM, 32QAM, or 64QAM.
 55. The system of claim 50, where the firstupstream channel comprises a first time division of the first frequency.56. The system of claim 55, where the second different upstream channelcomprises a second time division of the second different frequency. 57.The system of claim 50, where the quality comprises at least one ofbit-error-rate or signal-to-noise ratio.
 58. A method comprising:transmitting, by a modem, on a first upstream channel on a firstfrequency; receiving, by the modem, a command to select differentupstream transmission characteristics; selecting, by the modem, thedifferent upstream transmission characteristics in accordance with thecommand; and transmitting, by the modem, on a second different upstreamchannel on a second different frequency using the different upstreamtransmission characteristics.
 59. The method of claim 58, furthercomprising: receiving a plurality of messages, each message describingdifferent transmission characteristics.
 60. The method of claim 59,where the command indicates use of one of the plurality of messages forselecting different upstream transmission characteristics.
 61. Themethod of claim 59, where the first upstream channel comprises a firsttime division of the first frequency and the second different upstreamchannel comprises a second time division of the second differentfrequency.